Thermionic vacuum tube circuit



Sept. 29, 1936. BAKER 2,055,921

THERMIONIC VACUUM TUBE CIRCUIT Filed March 11, 1935 Inventor Patented Sept. 9, 1936 UNITED STATES.

P'ATENT- OFFICE 3 2,055,9z1 'rnnamomc vacuum Tuna cmeurr Paul Baker, Los Angeles, assignor to Robert A. Casey, Los Angeles, Calif.

Application March 11, 1935, Serial No. 10,454

I (Cl. 179-1) This invention relates broadly to vacuum tube circuits employing heater type tubes in which the cathodes of the tubes are not heated directly by the passage of current therethrough but instead 5 are raised to electron emitting temperature by heat conducted and/or' radiated from a separate electrically heated element positioned adjacent thereto.

It is characteristic of heater type tubes that there is a substantial time lag following application of current to the heater element before the cathode attains electron emitting temperature and becomes operative. This lag is objectionable in many devices, such as radio sets and intercommunicating telephone apparatus for example, because the user must wait for the tubes to heat before he can use the device. It is also objectionable because in many circuits excessive plate potentialis applied to the tubes by the power supply system while the tubes are heating, due to the fact that the tubes draw little or no plate current during the heating period.

A broad object of the present invention is to reduce the time lag referred to in devices employing heater type tubes.

A feature of the invention is a circuit for A. C. operated vacuum tube amplifiers and the like which at all times supplies a small amount of current to the heater elementsof the tubes to preheat them and yet .does not involve expensive, delicate or unreliable control apparatus and does not draw excessive energy from the line.

'In essence, my circuits constituting the present invention differ from prior circuits known to me in that they include a means for delivering a reduced current tothe heater elements of the tubes during inactive periods while at the same time preventing wastage of current in other portions of the circuit.

Thus it is present practice in vacuum tube amplifying devices adapted to be completely energized by alternating current from a light socket to, provide in the device a power transformer having a primary winding adapted to be con- .45 nected to the light socket, a low potentialsecondary winding or windings connected to the heater elements of the tubes, a high potential low potential alternating current to the rectifier filament; the device being turned on and off by simply making and breaking the power supply connection to the primary winding of .the transformer. Such a device, if modified in accordance with my invention, would include a current limiting device for limiting the potential applied by the firstmentioned secondary winding to the heater elements of the tubes, with a switching 5 device operable into on and off positions for applying reduced potential to the heater elements and opening the rectifier filament circuit when the switch is in "01? position and applying full potential to the heater elements and completing l0 the rectifier filament circuit when the switch is in "on". position.

I am aware that it is old to condition vacuum tubes for instant use by applying reduced current to the cathode heater elements during inactive l5 periods and-I am also aware that this reduced current has been obtained by reducing the potentials developed in the secondary windings of the power transformer. The practice of reducing the potentials developed in the secondary windings of the transformer is unsatisfactory, however, in

the circuits employing a hot cathode rectifier "ment but also from the high potential secondary winding supplying current to be rectified. I have 35 also found that rectifier filaments in such a circult have their lives very much reduced. My circuit, on the other hand, eliminates wastage of current, since no current whatever is drawn from the low potential filament heating winding and 40 the high potential winding supplying current to be rectified.

For purposes of illustration, the invention is disclosed ass-incorporated in an intercommunieating telephone system employing a vacuum tube amplifier. A full understanding of the invention, together with other objects and features thereof not heretofore mentioned, may .be had from the. following detailed description which refers to the drawing.

In the drawing: t

Fig. 1 is a schematic circuit diagram of an intercommunicating telephone system incorporating a modification of the circuit shown in Fig. 1; and

Fig; 3 is a detail schematic diagram illustrating still another modification of the circuit shown in Fig. 1.

Referring to Fig. l, I have shown two telephones I and 2, respectively, adapted to be interconnected through a vacuum tube amplifier, with control switches for energizing and deenergizing the amplifier and reversing the direction of communication. The telephones l and. 2 may be of any conventional form, although I prefer to employ as these units dynamic speakers of the permanent magnet type, in which no external source of current is required to energize the held in which the moving coil attached to the diaphragm is suspended. Inasmuch as this particular type of telephone unit is not essential, it has not been illustrated in detail, the construction of such devices being well known. 7

The amplifier disclosed is a three-stage unit containing three tubes 3, 3 and 5, respectively, of the heater type, each tube containing a cathode 6 adapted to be heated by conduction and/or radiation from a heater element F, a grid 3 and anode 9. The grid of the first tube 5 is connected to one end of the secondary winding of an input transformer Hi, the other end of which winding is connected to ground through a ground conductor H. A variable resistance I2 is shown connected in shunt across the secondary winding of input winding [0 for volume control purposes although if desired any other known method of varying the potential applied to the grid of the first tube may be employed. The cathode 6 of each of the tubes 3, 4 and 5 is connected to the ground conductor H through a grid biasing resistor l3 shunted by a by-pass conductor id. The amplifier is of the resistance coupled type, the anode of the first tube 3 being connected to a source of anode potential through a resistor till and coupled to the grid of tube 6 through a coupling condenser l5. Likewise the anode of tube 4 is connected to the source of anode potential through aresistor 80 and to the grid of tube 4 through a coupling condenser 85, and the grids of both tubes 4 and 5 are connected to the ground conductor ll through leak resistances it. The anode of the last tube 5 is connected through the primary winding of an output transformer H? to the source of anode potential.

The amplifier so far described is of well known construction and may be varied in design without producing any eifect insofar as the present invention is concerned. It is adapted to be energized by supplying alternating current of suitable potential to the heater elements 1 of the tubes and,

supplying constant potential of suitable value to the various leads l8, l9 and 20 connected to the anodes or the three tubes. To supply these various potentials, a power supply unit is provided,

rim

preferably as an integral part of the amplifier.

The power supply unit comprises as its essential elements a power transformer II, a hot cathode rectifier tube 22, and a filter 23. The power transformer 2| has a primary winding 24 adapted to be connected through an impedance element 25 directly to a suitable source of altemating current of constant voltage, such as a 110- volt lighting circuit. The transformer 2| is also provided with a low potential secondary winding 26 which is permanently connected through leads 21 and 28. to the heater elements 1 of the three tubes, ahigh potential secondary winding 29 having a mid tap ll connected to the ground conductor It and having its opposite ends connected through leads 3i and 32, respectively, to the two anodes of a full Wave rectifier tube 22, and a third low potential secondary winding 33 adapted to be connected through a contact 3 and armature 35 of a relay 36 to the filament of the rectifier tube 22. The filament circuit of the rectifier tube is also conected by a lead 37 through a resistor 38 to the high potential side of the filter 23, this filter comprising a reactance 39 and resistance elements at and at, all connected in series, and with condensers d2 connected between ground and the terminals of the reactance 39 and resistors fill and $5. The resistors 40 and it serve not only to filter the current flowing therethroug'n but to reduce its potential to suitable values for the tubes 3 and 3. Thus the anode supply conductor 26 for tube 5 is connected directly between reactance 39 and resistor 40, the anode supply conductor i9 for tube l is connected between the resistors ill and 3 l and the anode supply conducttor it for tube 3 is connected to the end of resistor ii. A potentiometer it; is also provided, connected across the first condenser E2 of the filter circuit for supplying a direct current to up crate a communication reversing relay 46. Thus the winding of relay M is connected in series, through a lead 55, between a tap "46 on potentiometer 33 and one contact ll of a switch 58 having a cooperating contact &9 connected to the ground conductor ll. Relay M is provided with a pair of armatures 5B and 5i, respectively, cooperating with front contacts 52 and 53, respectively, and back contacts E i and 55, respectively. The front contact 52 and the back contact 55 are connected together and to one terminal of the telephone l, the other terminal? of which is grounded. Similarly, the front cbntact 5 3 and back contact 53 are connected together and to one terminal of the telephone 2, the other terminal of which is adapted to be connected through a lead 56 and switch contacts 5i and 58, respectively, to ground.

The impedance element 25 connected in series with the primary winding 2 of the power transformer 2i has its two ends connected to an armature 55 and associated front contact 66 on relay 3t. Relay 3B is adapted to be operated by alternating current and has one end of its winding connected through the secondary winding of a transformer ill to ground and the other end of its winding connected through a lead 62 to a contact 63 on switch 6 5. Contact 63 is adapted to be SI for supplying current to energize relay 36 is permanently connected to the 110-volt A. C. supply line, as shown.

As shown in Fig. 1, the system is in idle condition and the impedance element 25 limits the potential across the primary winding 24 and the potentials developed in the secondary windings 25, 29 and 33 to values substantially below their normal values, depending upon the resistances of the various elements in the system. The impedance 25 is preferably chosen of such a value, with reference to the values of the other elements of the system, that itlimits the potential applied by the winding 26 to the heater elements 1 of the amplifier tubes to a value sufficient to maintain the cathodes 6 of the tubes just slightly below the temperature at which they emit electrons.

During idle condition no current is drawn from 25 ductor n.

45 former.

windings 29. and '33 for the reason that the circult of winding33 is open at contacts 34 and 35 on relay 36 and the filaments of the rectifier tube 22 are cold, thereby preventing any fiow of space current through the rectifier tube. It will prevent unnecessary losses in the impedance element 25, it is desirable that this impedance constitute a reactance or choke coil. However, in

many instances where initial cost is of more importance than operating cost, the impedance element may be constituted by a resistor.

Assuming that it is desired to place the system in operation, the switch 64 is thrown into on I position, in which contact 51 closes on contact 58', and cbntact63 closes on contact 65. Closure 20 of contacts 51 and 58 completes a circuitfrom ground through contacts 58 and 51, conductor 56,

telephone 2, back contact 53, and armature 5| of j relay 44, through the secondary winding of the output transformer I1, back to ground'on con- A circuit was previously completed from ground to the telephone I, back contact 52 and armature 50 of relay 44, through the primary winding of the input transformer Ill to ground on conductor H. Therefore, the telephone i will be conditioned to transmit and the telephone 2 conditioned to receive as soon as the amplifier is energized.

Energization of the amplifier is caused by the closure of contacts 63 and 65 on switch 64. Thus the closure of contacts 63 an'd 65 completes a cirnect the filament of the rectifier tube 22 to the secondary winding 33 of the power transformer, and closing contacts 59 and 60 to shunt out the impedance element 25 and apply full line potential to the primary winding 34 of the power trans- The increase in the potential applied to the primary winding 34 correspondingly increases the potential developed in the secondary windings 26, 29 and 33, the increased potential from winding 26-bringing the heater elements I of the tubes up to full heat and also immediately raising, the associated cathodes 6 to full electron emitting temperature, and the .winding 29 supplying full potential to the rectifier 22, thereby supplying full plate'potential to the amplifier tubes.

75 of the amplifier. The use The system disclosed is adapted for communication in only one'direction at one time. However, the direction of communication may be reversed, underthe control of the user. of the telephone 2,

by simply actuating the reversing switch 48 to close contacts 41 and 49. Closure of these contacts supplies current from the rectifier through thepotentiometer 43 and the lead 45, through the winding of relay 44, and through lead 66, contacts 41 and 49 of relay 48, backtothe ground conductor I I, which constitutes the other side of the rectifier circuit. Relay 44 thereupon pulls, breaking the armatures 56 and 5| from their back contacts 52 and 53, respectively, and connecting them .70 with their front contacts 54 and 55, respectively,

again reverse the direction of communication by opening the switch 48, thereby releasing relay 44 and restoring the circuits to their original condition as shown in the drawing.

A communicating system of the type shown is very sensitive to disturbances in the tube supply and switching circuits. To prevent undesirable clicks in the telephones at the time the switch48is opened or closed, a condenser 61 is shunted across the contacts 41 and 49. The use of the potentiometer 43 limits variations-in the potential applied to the anodes of the tubes in the amplifier when the connection to relay. 44 is made or broken and also aids in preventing arcing or sparking at the contacts 41 ahd 49 and the resultant disturbances in the telephones.

When the conversation is finished, the system is restored to idle condition by throwing the switch 64 into "o fi position (the position shown in Fig. 1). The breaking of the connection between contacts 63' and 65 opens thea'ctuating circuit for relay 36, thereby opening the contacts of that relay and breaking the filament circuit of the rectifier and restoring the current limiting lmplifier tubes require some time to cool below an.

electron emitting temperature and the condensers 42 of the filter system store a substantial amount of current, the telephone system would still be alive for an appreciable interval after contacts 63 and 65 on switch 64 open. This is undesirable and sometimes annoying and is prevented by completing the circuit from telephone 2 to ground through contacts 51 and 58 on relay 64. By the use of these contacts, however, the telephone circuit is opened immediately when switch 64 is thrown into oif position, thereby making the telephones "dead immediately. The method of reducing the voltage applied to the heater elements of the tubes comprising an impedance element 25 in series with the primary winding 24of the primary transformer is preferred because of its low cost and the fact that it may be employed with transformers of conventional design. However, there are other methods of obtaining the same result, two of which are disclosed-in the detail diagrams of Figs. 2 and 3, in which all parts corresponding to those in Fig. 1 bear the same reference numerals with the suffix a in the case of Fig. 2 and sufiix h in the case of Fig.3. 1 In the modification shown in Fig.- 2, the pri mary winding 24a is supplemented by an additional winding 68 connected in series therewith,

and the relay 36a, in addition to the armature 59a and front contact 60a, is provided with a back contact 69. The portion. 24a of the primary winding constitutes the usual or working winding and is connected directly across the supply circuit through contact 60a and armature 59a when the relay 36 is actuated. However, when relay 36 is released, as shown in Fig. 2, supplementary winding 68 is connected in series with the winding 24a across "source of supply through the back contact 69;. armature 59a. Obviously if the transformer 'is so designeckthat the secondary winding 26a delivers normal heating current to the heater elements of thefamplifier tubes when the winding 24a is connected directly across the' source of supply, thenwhen the additional winding 68 is connected in series with winding 241: the potential developed in the winding 26:: will be reduced in degree proportional to the increase in the number of turns in the primary winding. Therefore, by suitably proportioning the winding 68 with reference to the winding 2411, the potential developed in the winding 26a may be reduced to any desired value when the extra winding 68 is cut in. Since the winding 68 would never be required to carry a very heavy current, it can be made of much finer wire than the winding 24a.

In the modification shown in Fig. 3, the primary winding 24b is permanently connected to the source of supply and the potential delivered to the heater elements of the tubes over conductors 21b and28b is reduced by connecting those leads across only a portion of winding 26b when the system is idle. To this end, the relay 26b is provided with a back contact 69b which is connected to a tap 10 on the winding 26b, the front contact 60b of relay 36b is connected to the end of winding 26b and the armature 59b of relay 36b is connected to the end of conductor 21b. It will be readily apparent that with this arrangement when relay 36b is released the conductors 21b and 28b leading to the heater elements of the amplifier tubes will be connected across only a portionof winding 2617, thereby reducing the potential applied to the heater elements to a value sufiicient to maintain the associated cathodes at the desired temperature slightly below that at which they emit electrons. Except for the changes noted, the modified circuit shown in Fig. 3 corresponds exactly to the circuit of Fig. 1.

It will be apparent in the circuits shown in Fig. 1 and the modifications shown in Figs." 2 and 3 that the cathodes of the tubes 'will be preheated during idle periods without undue wastage of current since no current will be drawn from the high potential secondary'winding 29 or the cathode filament heating winding 33 and, except for slight losses in the transformer and in the impedance element (in the modification of Fig. 1) no electrical energy is consumed except that which-is actually used to preheat the heater elements.

It will also be apparent that I have provided an intercommunicating telephone system adapted to be completely, energized from a source of alternating current which system is rendered instantly active by closing the control switch and is rendered inactive instantly in response to opening of the control switch; and in which the direction of transmission may be reversed at will through the medium of a relay actuated by current from the rectifier system that supplies current to the anodes of the amplifier without introducing objectionable noises and without producing serious variationsin the potential ap plied to the anodes of the amplifier tubes.

aosaeai tuated mechanically by the switch 48. It is to be understood that the speakers I and 2 may be located at widely separated points and both widely separated from the amplifier, current supply apparatus and relays, or the circuits and apparatus relating to the preheating of the amplifier cathodes and control of the rectifier filament circuit may be incorporated in a single box or cabinet.

Furthemore, it will be apparent to those skilled in the art that my method of preheating the cathodes of tubes is in no sense limited to an intercommunicating telephone system, or to an audio frequency amplifier. Thus my method of preheating heater-type cathodes is equally applicable to and has a wide field of use in radio broadcast receivers, which may include oscillator, detector, and radio frequency amplifying tubes in addition to audio frequency amplifier tubes. In brief, my method of preheating is applicable to any vacuous tube device employing one or more heater-type tubes, and a hot cathode rectifier for supplying anode potential to the tubes.

I claim:

l. A vacuum tube device including a work circuit containing a working vacuum tube having an-anode, an indirectly heated cathode and a heater element for the cathode; and a power supply circuit for said work circuit containing a rectifying vacuum tube having a directly heated cathode, a power transformer having a primary winding adapted to be connected to a source of constant potential alternating current, a secondary winding connected to said heater element of said working vacuum tube for supplying heating current thereto, a secondary winding for supplying heating current to said rectifier cathode, and still another secondary winding for supplying high potential current through said rectifier tube to the anode of said working tube;

in combination with means for energizing and deenergizing said working circuit and preconditioning it for instant use, comprising current limiting means normally connected in the primary circuit of said power transformer for limiting the potential delivered by said first mentioned secondary winding to said heater element to a subngrmal value, and a power control switch means for simultaneously applying the full po-' tential of said source of said alternating current to said primary winding and connecting said rectifier filament to its associated secondary winding.

2. Vacuum tube apparatus comprising a work circuit containing a working vacuum tube having an anode, an indirectly heated cathode and a heater element for said cathode, in combination with an energizing circuit for said vacuum tube, said energizing circuit comprising a vacuum tube rectifier having a filamentary cathode, a power transformer having a primary winding, a first secondary winding connected to said heater element, a second secondary winding for supplying heating current to said rectifier filament, and a third secondary winding connected through said rectifier to the anode of said working tube, a source of alternating current of constant potential for energizing the primary winding of said transformer, means for varying the potentials induced in the secondary windings of said transformer between a high and a low value, control means operable between two positions for conditioning said potential varying means to develop high potential in said secondary windings and connect said second secondary winding to said rectifier filament in one position and for conditioning said potential varying means to develop a low potential in said secondary windings and disconnect said second secondary winding from said rectifier filament in its other position of operation.

3. In combination, a work circuit containing a vacuum tube having an anode, an indirectly heated cathode and a heater element for said cathode; an energizing circuit for said vacuum tube, said energizing circuit comprising a vacuum tube rectifier having a filamentary cathode, a power transformer having a primary winding, a first secondary winding connected to said heater element of said working tube, a second secondary winding for supplying heating current to said rectifier filament, and a third secondary winding connected through said rectifier to the anode of said working tube, a source of alternating current of constant potential for energizing the primary winding of said transformer, voltage varying means for varying the potential applied from said first secondary winding to said heater element, means for connecting and disconnecting said rectifier filament to said second secondary winding, and a single control means for simultaneously conditioning aid voltage varying means to apply full potential to said heater element and connect said rectifier filament to said secondary winding, or condition said voltage varying means to apply reduced potential to said heater element and disconnect said rectifier filament from said second secondary winding at will.'

4. Apparatus as described in claim 2, in which said means for varying the potentials induced in the secondary windings of said transformer between a high and a low value comprises an impedance element connected in series with the primary winding of said transformer with contact means for shorting out said impedance element, other contact means for connecting said secondary winding to said rectifier filament, and a single means for actuating both said contact means.

5. Vacuum tube apparatus comprising a work circuit, containing a working vacuum tube having an anode, an indirectly heated cathode and a heater element for said cathode, in combination with an energizing circuit for saidworking vacuum tube, said energizingcircuit comprising a vacuum tube rectifier having a filamentary cathode, a. power transformer having a tapped primary winding, a first secondary winding connected to said heater element, a second secondary winding for supplying heating current to said rectifier filament, and a third secondary winding connected through said rectifier to the anode of said working tube for supplying space current thereto. a source of alternating current of constant potential for energizing the primary winding of said transformer, control means for energizing and de-energizing said apparatus comprising a multi-blade switching device operable into two positions and adapted to connect said source across the full primary winding and open said rectifier filament circuit in one position and connect said source directly across a tapped portion or said transformer primary winding and complete said rectifier filament circuit in the other position.

6. Vacuum tube apparatus comprising in combination a work circuit containing a working vacuum tube having an anode, an indirectly heated anode and a heater element for the cathode; an energizing-circuit for said vacuum tube comprising a vacuum tube rectifier having a filamentary cathode, a power transformer having a primary winding, a tapped secondary winding adapted to be connected to said heater element of said working tube, a second secondary winding for supplying heating current to said rectifier filament and a third secondary winding connected through said rectifier, to the anode of said working tube for supplying space current thereto, a source of alternating current of constant potential connected to the primary winding of said transformer for energizing the transformer, control means operable between two positions and comprising a plurality of sets of contacts for simultaneously connecting said heater element across a tapped portion of said first secondary winding and disconnecting said rectifier filament from said second secondary winding in one position of operation and connecting said heater element directly across said first secondary winding and connecting said cathode filament to said second secondary winding in the other position.

7. In an intercommuni'cating telephone system, a pair of telephones, a vacuum tube amplifier connecting said telephones for receiving and amplifying voice currents generated by one telephone and applying them to the other telephone, said amplifier containing vacuum tubes having anodes, grids, and heater type cathodes of considerable mass, whereby they heat and cool slowly, power supply means for energizing said amplifier from a source 01' alternating current, said means including a rectifier and filter-for supplying direct current to the anodes of the tubes, the filter containing condensersj storing sufilcient electricity to energize the tube anodes for a substantial interval after current supply thereto from said rectifier has been cut oil, said means also including a transformer for supplying current to heat said cathodes, energizing switch means for said system comprising a member operable into on" and ofi positions and having a plurality of sets of contacts operable in response to movement of said member, some 01' said contacts completing circuits to supply rectified current to said filter and full heating current to said cathodes when said member is in "on" position and tointerrupt the supply oi rectified current to said filter and diminish the heating current to said cathodes when the said member is in ofi position, and others of said contacts completing voice current connection between id telephones through said amplifier when id member is in on position and breaking s d voice current connection when said member is in off position, whereby movement oi. said member into "ofi position renders said telephones inoperative instantly regardless of lag in the shut-down of said amplifier.

\ PAUL BAKER. 

